1. Field of the Invention
The present invention relates to transducers. More specifically, the invention relates to an improved transducer arrangement for low frequency sonar projectors that convert electrical signals to mechanically generated acoustic signals.
2. Description of the Prior Art
Transducers are employed as part of sonar devices which are used to detect underwater objects. Such transducers may be either a projector or a receiver. A projector is a sonar transmitter which converts electrical signals to mechanical vibrations, while a receiver conversely intercepts reflected mechanical vibrations and converts them into electrical signals. Projector transmitter and receiver arrays are formed from multiple projectors and receivers which are then utilized in conjunction with a sea craft to detect underwater objects.
A projector comprises an electromechanical stack of ceramic elements having a particular crystalline structure. Ceramic projectors must be operated in an optimal temperature range to provide good performance. Depending on the ceramic crystal structure, a projector may be either piezoelectric or electrostrictive. If the ceramic crystal is subjected to a high direct current voltage during the manufacturing process, the ceramic crystal becomes permanently polarized and operates as a piezoelectric. An electrical signal then applied to the ceramic stack generates mechanical vibrations. Alternatively, direct current voltage can be temporarily applied to the ceramic stack during operation to provide polarization of the crystal. Under these conditions, the operation of the projector is electrostrictive. After the application of the direct current voltage is discontinued, the electrorestrictive ceramic stack is no longer polarized.
Many different types of sonar projectors are known. One type of projector is a flextensional sonar projector which comprises a low frequency transducer. A low frequency transducer exhibits low attenuation of the acoustic signals in sea water. In general, a ceramic stack is housed within an elliptical-shaped outer projector shell. Vibration of the ceramic stack caused by application of an electrical signal produces magnified vibrations in the outer projector shell. Thereafter, the vibrations generate acoustic waves in the sea water.
Present mobile surveillance systems employ large, heavy arrays of low frequency high power Class IV flextensional transducers to provide the required source level, directivity, and bandwidth. High temperature Lead Magnesium Niobate (PMN) ceramics in a flextensional transducer are capable of developing much greater levels of voltage induced strain than prior art transducers, thus producing higher source levels of output from a projector. Replacement of driver material in a flextensional transducer can therefore be used to increase the power level without affecting the resonant frequency or frequency bandwidth of the device. Because PMN is a ferroelectric material it must be biased with a DC voltage during operation. Such flextensional sonar projectors require a large voltage DC bias capacitor to isolate the high voltage DC from a power amplifier and pass the high voltage AC which drives the transducer. Previous implementations of PMN driven transducers have used a bank of blocking capacitors to isolate a DC bias voltage from the AC drive voltage. The blocking capacitors are large and expensive, weighing as much as 30% of the transducer and must be physically located near the transducers. In order to accommodate evolving needs, smaller and lighter weight projector arrays are required.
The invention provides an improved push-pull transducer arrangement and an improved drive circuit which eliminates the need for the isolation capacitor. The transducer arrangement provides two attached transducers which utilizes a split bias technique to eliminate the heretofore required capacitor. The transducers operate out of phase from each other electrically but in phase with each other acoustically. Each transducer has approximately the same impedance over the operating band to create a balance of power output. In effect, the invention eliminates the blocking capacitors by utilizing two electrically out-of-phase transducer drivers to bias one another. The two drivers are used in a "push-pull" configuration within two different shells which have slightly different resonant frequencies. This coupled dual resonant system also significantly increases the frequency bandwidth of the transducer arrangement. Thus, an improvement in system size and weight is attained by eliminating the capacitors and a significantly increasing bandwidth is also achieved. Such a reduced weight, broad bandwidth transducer arrangement significantly reduces the size and cost of low frequency projector systems.